1. Field of the Invention
The present invention relates to a method and apparatus for controlling the air-fuel ratio in an internal combustion engine, for example, in an automatic vehicle.
2. Description of the Prior Art
In the prior art systems for air-fuel ratio feedback control in an internal combustion engine, the constant altering (so-called "dithering") of the flow rate of the bypass air, which bypasses a series connected air flow sensor and the throttle valve is carried out in such a manner that the air-fuel ratio is changed alternately from a basic or fundamental ratio at predetermined time periods to a rich setting and to a lean setting. The decision of the direction in which to change the basic or fundamental air-fuel ratio, to improve the specific fuel consumption, is made on the basis of this constant dithering, and correction of the basic or fundamental air-fuel ratio is made in accordance with this decision. That is the prior art systems dither the A/F ratio by making minor changes is the actual air flow. The performance of the engine is then monitored to determine whether the change in actual air flow improves or degrades engine performance. The system then adjusts the basic or fundametnal A/F ratio in accordance with this determination to improve specific fuel consumption. A prior art air-fuel ratio control system of this type is disclosed in Japanese Unexamined Patent Publication (Kokai) no. 57-124051.
However, in such a prior art air-fuel ratio control system, the flow rate of the air passing through the air flow sensor is either changed or not changed, depending on the existence or non-existence of the bypass air flow bypassing the series connected air-flow sensor and throttle valve, so that the actual flow rate of the fuel is not always constant. That is, the prior art systems assumed that the dithered A/F ratio holds the amount of fuel constant. Such is not the case. The flow of air through the bypass air path has a significant effect on the amount of air flowing past the air flow sensor. Reacting to this change in air flow, the prior art systems actually altered the fuel quantity in order to try and maintain the basic A/F ratio. Thus, when dithering changed the engine performance, prior art systems could not determine if the change in engine performance was due to the altered air flow or the altered fuel quantity or both. Prior art systems assumed that the change in engine performance was attributable solely to the change in air flow. Thus, when such systems made a decision to alter the basic A/F ratio to improve specific fuel consumption, that decision was based on uncertain data. Hence, it is difficult to correctly decide the direction in which the change of the air-fuel ratio should be made to improve the specific fuel consumption. This gives rise to the problem wherein the air-fuel ratio cannot be properly controlled in order to attain the optimum specific fuel consumption.